Improve error message when segmentation contains nodes not in graph

[cmalinmayor]

Currently, when loading tracks where the segmentation contains node IDs that don't exist in the graph, the error message is unclear:

Failed to load tracks: object of type 'NoneType' has no len()

This happens when the graph is a subset of the segmentation, which can occur in valid scenarios:

• User manually tracked only some cells in the segmentation
• Tracking algorithm filtered certain objects (by size, confidence, etc.)
• User deleted some tracks from the graph but underlying segmentation still has those objects

Proposed improvement:

Instead of a cryptic NoneType error, the code should:

1. Detect when segmentation has IDs not in the graph
2. Either warn and skip those IDs (if this is a valid use case), or
3. Provide a clear error message like "Segmentation contains node X which doesn't exist in the graph"

Context:

Discovered during testing of PR #231: #231 (comment)